FIG. 10 is a cross-sectional view showing a structure of a conventional dispersion shifted photonic crystal fiber. In FIG. 10, the reference numeral 21 designates a core section, 23 designates a cladding section, 24 designates a hole and 25 designates a jacket section. The holes 24 in the cladding section 23 are not located at random, but have a honeycomb structure composed of regular hexagons which have a side length of Λ, and serve as a primitive lattice. Here, the diameter of the holes 24 is represented by d.
The optical fiber has an effective refractive index lower than the refractive index of the core section 21 because of the holes 24 in the cladding section 23 so that the mode guided through the core section 21 is confined and transmitted. To achieve wavelength dispersion characteristics that give zero dispersion at around the wavelength 1.55 μm in this structure, it is necessary for the waveguide dispersion due to the holes in the cladding section to compensate for the waveguide dispersion that compensates for the material dispersion of a material (SiO2 glass, for example) constituting the optical fiber. This is achieved by setting Λ=1.6 μm and d=0.8 μm, for example.
FIG. 11 is a graph illustrating the wavelength dispersion characteristics of the conventional dispersion shifted optical fiber. FIG. 11 shows that the zero dispersion wavelength is present near the wavelength 1.55 μm. In addition, unlike the dispersion shifted optical fibers widely used as a medium for optical communication at present, it is characterized by that the dispersion slope (the gradient of the wavelength dispersion when the horizontal axis is wavelength) is negative.
The conventional dispersion shifted optical fiber, however, has the following drawbacks.
A first drawback is that since the dispersion slope utilizes a negative region (in which the dispersion reduces as the wavelength increases), the confinement effect of the propagation mode is weak, and the loss increases with an increase of the wavelength. In addition, as for a structure that modifies part of the optical fiber as shown in FIG. 10 by doping GeO2 into the central portion of the core section so that a refractive index at that portion becomes higher than its periphery, it also has a drawback that the optical loss increases at longer wavelengths like the structure as shown in FIG. 10, when the zero dispersion is implemented by the effect of the structure dispersion in the cladding section.
FIG. 12 illustrates the loss wavelength characteristics of the dispersion-shifted fiber in this case (K. P. Hansen, et al., “Highly nonlinear photonic crystal fiber with zero dispersion at 1.55 μm”, OFC 2002, Post Deadline Paper, FA9 (2002)).
In FIG. 12, the loss increases sharply from about 1.45 μm toward its longer wavelength side, and becomes a very large loss of about 100 dB/km at 1.6 μm. Similar characteristics have been obtained by computer analysis. In addition, since the weak optical confinement effect can easily bring about microbending loss, it is difficult to construct a low loss transmission line by a cable using the conventional technique. Furthermore, since a bending loss can easily occur because of the same reason, it is difficult to place the fiber in a small diameter when using the present optical fiber as an optical component.
A second drawback is that since the conventional dispersion shifted optical fiber has a small core diameter of about 2.4 μm as compared with a commonly used single-mode fiber with the core diameter of about 10 μm, or a commonly used optical fiber with the core diameter of 8-10 μm, the splice loss with these fibers is large of about a few decibels.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a 1.55 μm band dispersion shifted optical fiber that has a low loss and low dispersion slope.